The present invention relates to a ball end mill with two cutting inserts, and to these cutting inserts. In particular, the invention makes it possible to use two substantially identical cutting inserts in the two insert seats of the ball end mill, and to prevent the inserts from being fastened incorrectly in the insert seats.
Ball nose end mills are used in many metal working applications including mold making where the inherent strength and milling advantages of a fully radiused cutting edge for the copy milling of a mold cavity is best demonstrated. Its ball shape allows the end mill to attack the mold cavity from any angle or direction. The cutting edge when generated through the end of the ball mill becomes helical with rotation. This promotes shearing of the metal to be cut in an efficient manner.
A conventional ball nose end mill includes a generally cylindrical end mill body having a generally hemispherical forward end portion. Two insert-receiving seats or recesses are formed in the forward end portion, diametrically opposite to each other. Two indexable cutting inserts are mounted in the two seats and secured thereto by means of clamp screws. Such a ball nose end mill is disclosed in, for example U.S. Pat. No. 4,883,391 and U.S. Pat. No. 5,221,162. One disadvantage with these known end mills is that they necessitate two different types of cutting inserts. This fact increases of course the complexity of the tool and makes it more complicated to assemble for the operator. Moreover, it increases manufacturing and storage costs, since two different cutting inserts have to be produced and stored.
A ball nose end mill is disclosed in JP-U-5-67414 having two identical cutting inserts. These can be turned by 180.degree. in the same insert seat, when one cutting edge has been worn out, and they may also be interchanged between the two seats. However, their cutting edges are entirely curved by a radius corresponding to the cutting radius of the tool. This gives the ball nose end mill a very limited cutting depth, which is unsatisfactory for many applications, such as for steep inclinations at die-making. In practice, the maximum possible cutting depth of this known toot equals its cutting radius. Another disadvantage of the entirely curved configuration of the inserts disclosed in JP-U-5-67414 is that the bearing side surfaces on the insert and in the seat are curved. This requires a very high degree of accuracy which is difficult and costly to obtain due to the required extensive grinding.
The cutting inserts of all the ball nose end mills according to the above acknowledged prior art have a positive axial inclination. It is true that this decreases the cutting forces, but on the other hand it further exposes the fragile front tip points of the inserts to the shock-like impact forces to which the inserts are submitted due to the fact that they operate intermittently. When the cutting inserts of a ball nose end mill has positive axial inclinations, the impact points will be at the front tips of the inserts.
Thus, the primary object of the present invention is to increase the possible cutting depth of a ball nose end mill, in comparison to the cutting depths achieved by the ball nose end mills of the prior art, while using two identical cutting inserts.
A further object of the present invention is to increase the resistance to impact forces on the cutting edges, in particular on the fragile tip portions of the cutting inserts.
Yet another object of the present invention is to overcome the problems with rounded bearing surfaces, while maintaining the basic concept of two substantially identical cutting inserts in the ball nose end mill.
According to one aspect of the present invention, a ball nose end mill includes a holder shaft having two dissimilar insert seats, and two substantially identical cutting inserts, each of the inserts being clampable in either one of the two dissimilar insert seats.
According to another aspect of the present invention, a holder shaft for a ball nose end mill includes two insert seats for carrying two substantially identical cutting inserts. Each of the two insert seats includes a first end and a second end, the first end being narrower than the second end, the first end of a first one of the two insert seats being disposed at a tip of the holder shaft, and the first end of a second one of the two insert seats being disposed at an end of the insert seat distal to the tip of the end mill.
According to yet another aspect of the present invention, a cutting insert for ball nose end mills includes a first and a second curved cutting edge and a first and a second substantially rectilinear cutting edge. The first curved cutting edge is shorter than the second curved cutting edge and the first substantially rectilinear cutting edge is shorter than the second substantially rectilinear cutting edge.